The invention relates to additives for electro-dipcoating baths which prevent the formation of surface irregularities (craters) in the stoved coating film.
It is known that in the manufacture of motor cars, sheet-metal coils are used to manufacture body parts or whole bodies by punching and shaping, riveting and welding. Conventionally, the metal sheets or sheet-metal parts are oiled or provided for example with drawing greases, lubricants or cutting oils to ensue that transport proceeds without friction, that the punches and presses are treated gently and so on. The blank bodies are cleaned and degreased only once they reach the pre-treatment zone, and thereafter the blank bodies are phosphated and the first coating layer is conventionally applied by the process of cathodic electro-dipcoating.
During stoving of this first coating layer in the stoving ovens (driers), the drawing greases and oils enclosed in the folds and flanges heat up and are sprayed out as a result of the pressure that develops. As a result of the circulating air flow in the stoving ovens, the droplets of oil and grease are distributed on the first coating layer, which has not yet been cross-linked, and produce surface irregularities, so-called craters.
The object of the invention was therefore to develop a surface-active substance (“anti-cratering”) which prevents the formation of craters and has no disadvantageous effects on the subsequent layers. These substances must be incompatible with the aqueous coating system, have low surface tension, spread out well on the (not cross-linked) coating surface, be easy to apply, and have no disadvantageous influence on the coating properties in the concentration used. Disadvantageous effects on subsequent layers are adhesion problems in the subsequently applied coatings, for example with plastics such as PVC, with adhesives, filler layers and refinishes. If for example silicone oils are added to the dipcoating bath, then although cratering is reduced the adhesion of the subsequent coating layers is impaired to the extent that satisfactory coating is not possible. The addition of certain resins to the bath liquid has also been recommended: in patent application JP-A-61-115974, an epoxy-amine adduct comprising an epoxy resin modified with dimeric fatty acids and a polyoxyalkylene polyamine is described. Although this does allow the tendency to cratering to be reduced, the adhesion of the subsequent layers is impaired. A further example of epoxy-amine adducts as an anti-cratering agent can be found in EP-A-0070550: although the adduct, comprising an epoxy resin and a polyoxyalkylene polyamine containing primary amino groups, reduces the tendency to cratering, it also has a disadvantageous effect on the adhesion of the subsequent coating layers.
Also described is the addition of homopolymers or copolymers of certain alkyl vinyl ethers to the dipcoating bath. In this case, the contents by mass of these polymers in the bath liquid are kept between 10 and 10000 ppm, preferably 150 to 500 ppm. It has been observed that when the content of polyalkyl vinyl ethers falls below the preferred range, craters are produced in considerable numbers, even without oils or greases being in the bath liquid. When the preferred range is exceeded, exudation occurs, in which the surface of the coating film is coated with a greasy layer. This also has a disruptive effect on the adhesion of the subsequent layers. It is therefore important to keep the content of this polymer within the preferred range. Because of the low quantity used and the difficulty of determining the proportion of this polymer in the bath liquid simply and quickly, the need arose to find an additive to suppress craters which has a broader range of application as regards its concentration in the bath and which enables the concentration in the bath liquid to be determined in a simple way.